Without limiting the scope of the present invention, its background will be described with reference to cementing a string of casing within a wellbore as an example.
In primary cementing operations carried out in oil and gas wells, a hydraulic cement composition is disposed between the walls of the wellbore and the exterior of a pipe string, such as a casing string, positioned within the wellbore. The cement composition sets in the annulus thereby forming an annular sheath of hardened impermeable cement therein. The cement sheath physically supports and positions the pipe in the wellbore and bonds the pipe to the walls of the wellbore, thus preventing the undesirable migration of fluids between zones or formations penetrated by the wellbore.
One method of primary cementing involves pumping the cement composition down through the casing and then up through the annulus. This method requires calculating the volume of cement required to fill the annulus. Once the calculated volume of cement has been pumped into the casing, a cement plug is placed in the casing. A drilling mud is then pumped behind the cement plug such that the cement is forced into and up the annulus from the far end of the casing string to the surface or other desired depth. When the cement plug reaches a float shoe disposed proximate the far end of the casing, the cement should have filled the entire volume of the annulus. At this point, the cement is allowed to dry in the annulus into a hard, impermeable mass.
Due to the high pressure at which the cement must be pumped, at a pressure above the hydrostatic pressure of the cement column in the annulus plus the friction pressure of the system, fluid from the cement composition may leak off into a low-pressure zone traversed by the wellbore. When such leak off occurs, the remainder of the cement composition near this low-pressure stops movement and quickly sets at that location in the annulus. Once this occurs, additional cement cannot be pumped past this location and all the cement in the system sets. Thereafter, remedial cementing operations, commonly referred to as squeeze cementing, must be used to place cement in the remainder of the annulus. In addition, a large mass of cement, which was intended to be placed in the annulus, must now be drilled out of the casing.
Accordingly, prior art attempts have tried to avoid the problems associated with fluid leak off into low-pressure zones during cementing operations. In one method of avoiding such problems, called reverse cementing, the cement composition is pumped directly into the annulus. Using this approach, the pressure required to pump the cement to the far end of the annulus is much lower than that required in conventional cementing operations. Thus, the likelihood of flash freezing the cement in the annulus before the entire annulus is filled with cement is significantly reduced.
With reverse cementing, it is necessary to identify when the cement begins to enter the far end of the casing such that the cement pumps may be shut off. Continuing to pump cement into the annulus after cement has reached the far end forces cement into the casing, which in turn may necessitate a drill out operation.
One method of identifying when the cement has reached the far end of the annulus involves running a neutron density tool down the casing on an electric line. The neutron density tool monitors the density out to a predetermined depth into the formation. When the cement begins to replace the drilling mud in the annulus adjacent to the neutron density tool, the neutron density tool senses the change in density and reports to the surface that it is time to stop pumping additional cement into the annulus. Another method of identifying when the cement has reached the far end of the annulus involves running a resistivity tool and a wireless telemetry system down the casing on a wireline. The resistivity tool monitors the resistivity of the fluid in the casing such that when the cement begins to replace the drilling mud in the casing, a wireless signal sent to the surface indicates it is time to stop pumping additional cement into the annulus.
Use of such retrievable tool systems can be prohibitively expensive. Neutron density tools and resistivity tools can also be ruined during such operations as a result of the cement entering the far end of the casing and contacting these tools.
Therefore, a need has arisen for a system and method for cementing the annulus between the wellbore and the casing that does not require pumping the cement at pressures that allow for leak off into low-pressure zones. More generally, a need has also arisen for a system and method that identify when to stop pumping fluids, including but not limited to cement, into the wellbore. Further, a need has arisen for such a system and method that do not require the use of expensive equipment including tools that must be retrieved from the well once the introduction of fluids is complete.